The interesting question is likely (IMO) to be that it can create liability. Selectively picking & ordering the content of others can trivially create new mean

The interesting question is likely (IMO) to be that it can create liability. Selectively picking & ordering the content of others can trivially create new meaning, for example the classic trope of a ransom note formed by clipping words & phrases out of a newspaper & gluing them onto a page. Whether a particular site's editorial actions on user content create liability is likely to be determined on a case-by-case basis. Real site's algorithmic recommendation systems aren't nearly as transformative as newspaper clipping ransom notes, but the sequence and organization of recommended content may well count as a new composite work (much like a music album is a work composed of a sequence of individual tracks) and create liability. I wonder if it would also qualify for individual copyright.